Liquid crystal displays are used in many applications where the orientation of the viewer with respect to the display is not fixed. While generally not a problem, the output of the LCD is linearly polarized. If the viewer is looking at the display through another medium that also is a linear polarizer (such as sunglasses), the image on the display can present artifacts, have portions which are partially or fully extinct, be distorted, or be simply unviewable.
Today, many liquid crystal displays also include a touch sensing layer that allows the device to be used as a touch-based input device. The touch sensing layer, if part of the optical stack defining the liquid crystal display, is selected based on both its touch sensing properties (such as a dielectric constant for a capacitive touch sensitive layer) and optical properties (whether it is clear). Commonly used materials for a touch-sensitive layer, such as polyethylene teraphthalate (PET), can exhibit high birefringence, an optical property used to describe an optical media which has an index of refraction that may depend on the direction of light propagating through it, such that there may be an extraordinary index as well as an ordinary index of refraction. Such effect of non-isotropic index of refraction may be introduced by crystalline lattice structure, such as in uniaxial crystals, or a result of internal stresses in the media. Whether the birefringence is localized in a pseudo random manner across position, or uniform and substantially constant over the layer, light transmitted through a partially or fully birefringent layer may undergo a phase delay between orthogonal components of a linearly polarized input state such that the light exiting the layer may experience a change in the polarization state, which may include conversion of linear input to partially elliptical, rotated, or a combination thereof. When the conversion is not consistent or fundamental for a range of input wavelengths, or further the conversion is somewhat randomized with respect to position, the result can be variation of not only luminance across the field of view, but further may induce variation in state differently for various wavelengths, thus may further cause colorations across the field of view.
If there is any material in the optical stack of the LCD that has high birefringence, the light output by the top layer, a linear polarizer, may cause a rainbow-like artifact to appear on the displayed image, depending on the orientation of the viewer with respect to the display. If the viewer is looking at the display through another medium that is also a linear polarizer (such as sunglasses), this rainbow effect can be exacerbated.